Paper coating compositions are generally prepared by forming a fluid aqueous suspension of pigment material together with a hydrophilic adhesive and other optional ingredients. Lightweight coated, or LWC, paper is generally coated to a weight of from about 5 g.m−2 to about 20 g.m−2 on each side, and the total grammage, or weight per unit area of the coated paper is generally in the range of from about 49 g.m−2 to about 70 g.m−2. The coating may conveniently be applied by means of a coating machine including a short dwell time coating head, which is a device in which a captive pond of coating composition under a slightly elevated pressure is held in contact with a moving paper web for a time in the range of from 0.0004 second to 0.01 second, before excess coating composition is removed by means of a trailing blade. However, other types of coating apparatus may also be used for preparing lightweight coated paper. LWC paper is generally used for printing-magazines, catalogues and advertising or promotional material. The coated paper is required to meet certain standards of surface gloss and smoothness. For example, the paper may have a gloss value of at least about 32, ae.g. up to about 50, TAPPI units, and a Parker Print Surf value is generally in the range of from about 0.5 μm to about 1.6 μm.
Ultra lightweight coated, or ULWC, paper is sometimes otherwise known as light lightweight coated, or LLWC, paper and is used for catalogues and for advertising and promotional material sent through the mail to reduce mailing costs. The coating weight is generally in the range of from about 3 g.m−2 to about 8 g.m−2 per side. The total grammage is generally in the range of from about 30 g.m−2 to about 48 g.m−2.
An important white inorganic pigment for use in preparing coating compositions for the manufacture of LWC and ULWC papers is kaolin obtained from kaolin clay. Large deposits of kaolin clay exist in Devon and Cornwall, England and in the States of Georgia and South Carolina, United States of America. Important deposits also occur in Brazil, Australia, and in several other countries. Kaolin clay consists predominantly of the mineral kaolinite, together with small proportions of various impurities. Kaolinite exists in the form of hydrous aluminosilicate crystals in the shape of thin hexagonal plates, but these plates tend to adhere together face-to-face to form stacks or booklets. The individual plates may have mean diameters of 1 μm or less, but kaolinite particles in the form of booklets or stacks of plates may have an equivalent spherical diameter (esd) of up to 10 μm or more. Generally speaking, kaolin clay particles which have an equivalent spherical diameter of 2 μm or more are in the form of stacks of kaolinite plates, rather than individual plates. Additionally, in some secondary kaolin deposits smaller kaolin particles may sometimes adhere together in random orientations to form agglomerated particles having equivalent spherical diameters significantly greater than 2 μm.
The kaolin deposits in England differ from those in the United States of America and Brazil in that the English deposits are of primary kaolin, while those in the USA and Brazil are of the sedimentary or secondary type. Kaolin was formed in geological times by the hydrothermal decomposition of the feldspar component of granite, and primary kaolin is that which is obtained directly from the granite matrix in which it was originally formed. On the other hand, secondary or (tertiary) kaolin also known as sedimentary kaolin has been washed out of the original granite matrix in geological times and has been deposited in an area remote from the site in which it was originally formed. Secondary kaolin deposits tend to have a higher proportion of fine particles, i.e., those having an esd smaller than about 2 μm, because the kaolin has undergone a certain amount of natural grinding during the course of its transport from its site of origin to its site of final deposition. See, for example, Jepson (Jepson, W. B., “Kaolins: their properties and uses”, Phil Trans R Soc Lond, A311, 1984, pp 411-432). Secondary kaolins also tend to be more blocky than primary kaolins, containing large numbers of both kaolin stacks and agglomerates of smaller blocky kaolin particles.
As long ago as 1939, Maloney disclosed in U.S. Pat. No. 2,158,987 that the finish, or gloss, of a clay coated paper is greatly improved if the clay, before incorporation in the coating composition, is treated so that a large percentage, for example 80% by weight or more, of the clay particles have a size in the range of 0.1 μm to 2 μm. In order to increase the proportion of fine particles in the raw kaolin, the raw kaolin may, according to the disclosure in U.S. Pat. No. 2,158,987, be subjected, before a centrifuging step, to a grinding operation in which a suspension containing from about 50% to about 75% by dry weight of kaolin and a dispersing agent is subjected to pebble milling. When the kaolin from the finer fraction is recovered, mixed with a suitable paper coating binder, and applied to the surface of a base paper, a coating of good gloss and color is obtained.
Various pigment products which are made using the principles described by Maloney in U.S. Pat. No. 2,158,987 are commercially available and provide good gloss and smoothness in coated papers, especially for LWC and ULWC paper. For example, a known pigment product available from Imerys, Inc., and recommended for gloss coatings of LWC comprises a refined English kaolin product having a particle size distribution, “psd”, such that 89% by weight of the particles have an esd less than 2 μm, 74% by weight of the particles have an esd less than 1 μm and 25% by weight of the particles have an esd less than 0.25 μm.
One particularly useful kaolin pigment product which is commercially available fromlmerys, Inc. for use in gloss papers is marketed under the tradename of ‘SUPRAPRINT™’. SUPRAPRINT™ is derived from a coarse, primary English clay and generally has a particle size distribution such that about 90% by weight of the kaolin particles therein have an esd less that about 2 microns and about 12-16% by weight of the particles have an esd less than about 0.25 microns. Further defining characteristics of the SUPRAPRINT™ kaolin include its shape factor in the range of 30-35, and its particle steepness of approximately 40. Due to its high steepness and shape factor and resultant particle packing effects, the opacity and gloss of SUPRAPRINT™ have proven particularly suited for use in gloss paper coatings and the like.
SUPRAPRINT™ is produced by from a Cornish primary clay having naturally platy fines-via the steps of: (1) pressing and pugging; (2) makedown into a slurry; (3) mild grinding to partially delaminate a portion of the kaolin stacks present; (4) fractionation using a decanter centrifuge; and a series of beneficiation and finishing steps. A portion of the coarse fraction from step 4 is sometimes recycled to grinding step 3 in an effort to maximize the yield of the process, but is not required to obtain product having the desired parameters. Though overall coarse in nature, the primary Cornish clay used has a high concentration of ultrafine platy particles which undesirably decrease the yield and increase the amount of waste material that must be disposed of from the production process. Accordingly, production of the SUPRAPRINT™ product from coarse primary deposit of English kaolin has proven to be undesirably costly.
It would clearly be desirable to provide for the production of a pigment product having similar parameters from a cheaper and easier to process secondary kaolin deposit that has a much lower quantity of ultrafine platy particles, such as can be found in the Rio Capim region of Brazil. Due to the inherently blocky nature of such secondary kaolins the required processing would likely need to differ significantly from that used to produce such a product from a more platy and coarse primary kaolin. Specifically, one might increase the shape factor of a blocky secondary kaolin through a harsher delamination that is typically used for the naturally platy primary kaolins. However, due to the presence of agglomerates of blocky kaolin particles in such secondary kaolins, one of ordinary skill in the art would expect typical delamination methods to also result in an undesirable decrease in shape factor as the agglomerates break up into fine blocky particles instead of delaminating.
We have now developed an improved kaolin pigment product and method of production thereof that addresses these shortcomings of the prior art. The present invention provides a pigment product having the desired gloss and/or brightness, particularly, but which can be easily and inexpensively produced from a secondary kaolin deposit. In addition, the pigment product of the present invention may result in a reduction of the conventional amount of alternate pigments, for example, titanium dioxide, that would be have to be used.